How trees can contribute to reconstruct the local pollution history and remove ultrafine particles from the atmosphere

Dendrochemistry has proven to successfully provide a temporal record of trace metal deposition from a variety of pollution sources. Trace element uptake by trees was mainly thought to occur by soil-root transfer but direct transfer of particulate matter from the atmosphere to the shoots is also possible. However, the physiological mechanisms involved in the uptake and transport processes of ultrafine particles is largely unknown. Their presence in the tree rings is still unexplored.
Measuring the chemical composition of the tree rings of Mongolian oak using laser ablation-ICP-MS analyses, we were able to reveal the pollution history (1965–2020) in Harbin, China. Through several greenhouse experiments, we wanted to understand the foliar and root uptake of particles with size smaller than 100 nm in forest tree species. Gold nanoparticles were applied on leaves and roots of beech and pine for a 2-months treatment. Our research showed that, besides trace elements, trees can take up and transport (via xylem or phloem pathways) engineered nanoparticles into their stem. These ultrafine particles were able to enter either the leaves, mainly through the stomata, or the roots through the root tips or microscopic cracks and eventually accumulate in the stem.
Dendrochemical analyses are a reliable tool to monitor the long-term history of pollution and contribute to extend instrumental records of pollution back in time. Moreover, the presence of nanoparticles in tree rings can be potentially explored by the use of dendrochemistry, that can assess their spatial and temporal distribution in areas close to pollution sources.